1. Field of the Invention
The present invention relates generally to a method for forming metal-free strips on a plastic sheet that is used to manufacture electrical capacitors and, more particularly, to a method using pulsed laser beams to remove a regenerably thin layer of vaporizable metal from a plastic sheet.
2. Description of the Prior Art
U.S. Pat. No. 4,462,062 discloses a method wherein a broad plastic sheet with a completely metallized surface is moved past a laser ablation station having a plurality of laser beams to form narrow demetallized strips. The plurality of laser beams are incident to the sheet in a line perpendicular to the direction of the sheet feed so that each laser beam forms a metal-free strip.
For manufacturing capacitors, broad, usually stretched, plastic sheets of polyethylene terephthalate, polycarbonate, polypropylene or other suitable dielectric material is provided with metal-free strips and is cut into narrow bands which have a demetallized free edge at one edge and which are processed into round or flat windings or which are layered in accordance with a known method.
The capacitor windings or stacks have contacts applied at the end faces by a metal spraying method so that all metallized layers which extend up to the edge of the end face are connected to one another in an electrically conductive fashion.
Instead of completely demetalizing the free edge of the dielectric in a region adjacent one end face, narrow plastic bands in the form of thin metal-free strips can be used to insulate the edge region of the metallized layers from the capacitively effective region of the metallization. For uniform windings and stacks, twice the number of metallized layers are available for contacting at each end face which thereby noticeably boosts the adhesion of the sprayed on metal contact layer. The metal-free strips can be extremely thin, for instance, for capacitors having a nominal voltage of 63 v., insulating strips having a width of only 0.1 mm have been found to be adequate.
Various methods are known for forming the exposed edges, or insulating strips, on thin metal-coated plastic sheets. For example, in German Patent 0S No. 3 224 234, a method is disclosed which largely preserves the sheet material having a thickness of only a few .mu.m which uses diaphrams entrained cover bands, on removable protective layers in an oil base. The oil base keeps defined strip-shaped regions of the sheet surface free of a metal during the metallization of the sheet by vapor deposition in a vacuum. In such method, the metal-free strips cannot be formed arbitrarily narrow. The lower limit for forming metal-free strips by such method is a width of approximately 1 mm. Another disadvantage of the above method is that residues of covering oil often remain in the edge regions of the strips that are formed and these oil residues work into the capacitor winding and change the capacitor properties over the long term.
Finally, broad plastic sheets which are already provided with metal-free strips during the metallization process can only be used for purposes that have been defined from the outset of the sheet manufacture.
Also known are methods for subsequently providing broad, completely metallized plastic sheets with metal-free strips. For example, in German 0S No. 2 348 904, a method is disclosed wherein circular burn-out disks or wheels having a thickness corresponding to the width to the strips to be burned out, and acting as burn-out electrodes, are resiliently pressed against the metallization of a metallized plastic sheet conducted in self-bearing fashion to cause topically limited destruction of the metallization by an electrical current. The metallization functions as the cooperating electrode to the electrically charged disk or wheel.
Such method of ablating undesired metal layers by a voltage carrying electrode is not without problems, particularly for thin plastic sheets since the extremely thin sheets are easily deformed or damaged at the ablated locations as a result of the sheet's thermoplastic properties.
Purely mechanical methods for removing the metal layer from the plastic sheets are also known, wherein the metal layer is ground off by rotating grinding wheels. In German Patent AS No. 2 509 543 is disclosed grinding wheels fashioned of bonded hard sharp edged grains of various degrees of fineness. The plastic sheet is pressed against the grinding wheels with a defined contact pressure by controlling the sheet tension.
As shown in German AS No. 1 938 320, the grinding wheels may be formed of rubber or silicone caoutchouc. The grinding wheels for removing regions of the metal layers are driven at high speeds and are arranged to press against a mating roll via an insulator band conducted therebetween.
The rubbing off, or abrasion of the metallized layers by grinding wheels of rubber or siliconed caoutchouc rotating at high speeds is easily accomplished and practiced, although it does involve some outlay due to the necessary pressure against the mating roll. However, with such method, there is the danger of overstretching and damaging the plastic sheets in the regions to be demetallized.
In the use of grinding wheels formed of hard, sharp edged grains, there is a risk of grinding through the sheet wherein a pressure roll can be omitted. Finally, as disclosed in the above-mentioned German AS No. 1 938 320, the width of the metal-free strips is limited by the thickness of the burnout or grinding wheels, for example, to a width of 0.3 mm.
While ablating the metal coating with an intense laser pulse is an extremely flexible method for generating metal-free strips on metallized plastic sheets, since the lack of physical contact is relatively gentle on the sheets, and thus avoids many disadvantages of the known methods. As set forth in the aforementioned U.S. Pat. No. 4,462,062, the laser pulse method has an added advantage that extremely narrow strips can be produced.
Generally, very high intensity laser beams present a problem in demetalizing thin plastic sheets. A continuous laser beam, or a pulsed laser beam impinging the same region of the sheet in quick succession, can not only evaporate the metal layer, but also heat, and even destroy, the thermoplastic material of the plastic sheet.
Not only is melting or evaporation of the sheet material a problem, but even at temperatures noticeably lower than the melting point of the plastic, the stretched plastic sheet is destretched. This results, for example, in folds and ripples appearing in the sheet material to the point that the sheet becomes unstable for manufacturing faultless capacitor windings and stacks. For this reason, temperature stable plastic sheets are processed by this method.